Continuation of Master Cylinder story

[66NICK98]

Well, today I bench bled the master cylinder to get all the air out of the piston which I thought was going fine until I noticed brake fluid leaking out of the rear of the piston.ad

This was the reason I had to buy a new one in the first place.

So, back it goes to Ecklers who are shipping me a new one.

Very frustrating because without the brakes working properly you obviously cannot drive the car.

Let's hope I have better luck with the new MC.

 

[BADBIRDCAGE]

When bench bleeding take care not to overtravel the piston. It is not meant to bottom out before you stop motion.

Rich

 

[66NICK98]

thanks Rich, does it ruin the MC if you do bottom out? How far should you depress the piston?

I was under the impression that full compression of the piston would be reflective of the
actual brake pedal rod doing the same. Sounds like this is not the case.

Thanks for your help.

NIck

 

[BADBIRDCAGE]

Nick: Bench bleeding is done to displace any air that might be introduced into the brake lines from the M/C. To bleed the M/C you shouldn't need more than about 3/4 of an inch of travel with the piston.

As I understand it what happens when you overtravel is that the piston "cup" actually passes/partially passes the supply hole from the MC reservoir to the piston and can pull fluid back with it on the return. This would account for brake fluid being pulled back with it.

Rich

 

[JohnZ]

As I understand it what happens when you overtravel is that the piston "cup" actually passes/partially passes the supply hole from the MC reservoir to the piston and can pull fluid back with it on the return. This would account for brake fluid being pulled back with it.

Rich

The cup on the piston always passes the supply hole or compensating port with brake application; the front edge of that cup goes past that hole with the first .030" of piston movement - that's what allows it to build pressure in the bore. When released, the piston returns to its rest position, and any fluid behind it passes back through holes in the piston to the main bore.

I've been full-stroking master cylinders while bench-bleeding for 40+ years, and have never had an issue.

 

[BADBIRDCAGE]

John:

I ran into the same problem Nick is having with the M/C on my son's 87. Leakage to the rear of the M/C while bench bleeding. Tried it again with a different M/C and didn't push to the bottom and didn't have the failure.

I certainly can't dispute your explanation.

Thanks

Rich